My guess would be no gain.
From what little I think I know... It seems the video speed isn't that great, OS2008 compensates by skipping every other or third frame. Fine by me but seems that would have been the greatest place for gain from high speed cards.
Possible exceptions could be simple data transfer speed, USB or wifi? Another thread I just saw hinted at booting from memory card for more speed. This would likely benefit.
EDIT: It seems I was right, except the OS kernel has a low clock speed for compatability. So, if you are going to figure out how to boot from flash, you would need a recompiled operating system.
Has there been any change for OS2008?